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Viscera, vessels and muscles at the fourth lumbar level.
Hey everyone! It's Nicole from Kenhub, and welcome to our cross-section tutorial at the level of the fourth lumbar vertebra. We'll begin this tutorial by reviewing the planes of the body and identify which one we'll use for cross-sectional images. We'll also briefly cover what a cross-sectional image is and how to orientate ourselves when looking at them. Next, we'll look at where the fourth lumbar vertebra is so we can anticipate what sort of structures to expect to see in the cross-sectional images at this level and finally, we'll get on to identifying these structures.
The structures that we'll be looking at include the muscles including those of the anterior abdominal wall, the muscles of the posterior abdominal wall and the muscles of the deep back. Next, we'll identify the organs at the L4 level then we'll find any visible nervous structures and finally, we'll identify the arteries and veins at L4. After we've identified all of the structures at L4, we'll chat about what you might see at this level clinically.
Let's begin by talking about the body planes.
Body planes are imaginary planes or flat surfaces but cut through and section the body while it's in the anatomical position. The first is the plane we can see highlighted in this image. This is the coronal plane. The coronal plane is a vertical plane that divides the body into anterior and posterior parts.
Now what we can see highlighted is another vertical plane but this time dividing the body into left and right parts. This is called the sagittal plane. The third and final body plane is the transverse plane which we can now see highlighted in green. This is a horizontal plane which divides the body into superior and inferior portions. Transverse planes are the planes we make cross-sectional cuts in so this is what's happened to the body when we look at cross-sectional images.
In this video, we'll be looking at some structures that we can see at this level in the transverse or cross-sectional plane. This cross-sectional images are similar to what can be seen in CT and MRI scans. Just as with those scans, in cross-sectional anatomy, we're looking at the images as if we're standing at the patient's feet looking superiorly towards the patient's head. Also, it should be noted that the patient is always lying on their back.
Therefore, over here is the anterior or front of the body and at the bottom of the screen is the posterior aspect. This side of the image is the right-hand side of the patient's body and this is the left-hand side where the patient's back is located.
So let's jump into identifying structures at the level of the fourth lumbar vertebra. The fourth lumbar vertebra is towards the inferior portion of the abdomen just superior to the pelvis, therefore, we can expect structures at this cross-sectional level to be parts of the distal and inferior portions of the digestive tract. In this cross-sectional image, the highlighted structure is the fourth lumbar vertebra.
The first set of structures we'll identify are the muscles at the fourth lumbar vertebral level. There are three groups of muscles that we'll look at – the anterior abdominal wall muscles, the posterior abdominal wall muscles and the deep back muscles. We'll begin with the anterior abdominal wall muscles and some of their features. In just a few moments, we'll take a look at the external and internal obliques, the transversus abdominis, the rectus abdominis, the rectus sheath, the linea alba, and the linea semilunaris.
The anterior abdominal wall is made up of four paired muscles. At this vertebral level, three of these muscles wrap around more laterally. The most external muscles are the muscles we can see highlighted in green. They're the external oblique muscles. The external oblique muscles are the most superficial of this muscle group. Anteriorly, they contribute to the rectus sheath which we'll look at later. When both of these muscles contract together, they flex the trunk. If one external oblique muscle is activated, it produces lateral flexion and contralateral rotation of the trunk.
Just deep to the external oblique muscles are the internal oblique muscles seen here. Similar to the external oblique muscles, the internal obliques contribute to the rectus sheath anteriorly. Bilateral contraction of these muscles also leads to flexion of the trunk. If only one internal oblique muscle is contracted, it leads to lateral flexion and ipsilateral rotation of the trunk.
The deepest of these muscles are the transversus abdominis muscles which we can now see highlighted in green. Again, anteriorly these muscles contribute to the rectus sheath. The main action of transversus abdominis is to help maintain posture and support the abdominal viscera.
The fourth and final pair of muscles contributing to the anterior abdominal wall are the rectus abdominis muscles. They run along the anterior aspect of the abdomen and are enveloped in the rectus sheath. The rectus abdominis muscles flex the trunk.
So, finally, let's look at the rectus sheath. This is the structure we can now see highlighted in green. It envelops the rectus abdominis muscles above the level of the arcuate line which is the level we're at now. The first three muscles we looked – external oblique, internal oblique and transversus abdominis – contribute to the rectus sheath. We can see the aponeurosis of the external oblique muscles here as they contribute to the anterior wall of the rectus sheath. The aponeurosis of the internal oblique muscles splits to run both anterior and posterior to the rectus abdominis. Finally, the aponeurosis of the transverse abdominis muscles contribute to the posterior wall of the rectus sheath.
In the midline of the anterior abdominal wall between the two rectus abdominis muscles, there is a long white tendinous line called the linea alba. The linea alba is highlighted in both images. This is formed by the fusion of the three aponeurosis from the external oblique, the internal oblique and the transversus abdominis muscles. Similar to the linea alba, there are two semilunar lines on the lateral aspect of each rectus abdominis muscle. These semilunar lines are also formed by the fusion of the three aponeurosis from the anterior abdominal wall muscles.
Next, we'll look at the two muscles contributing to the posterior abdominal wall. These are the psoas major and quadratus lumborum muscles.
There are two large muscles we can see here on either side of the vertebral body. These are the psoas major muscles which you can see highlighted in green. The psoas major muscles travel inferiorly and join with another muscle called iliacus to become the iliopsoas. The iliopsoas inserts onto the lesser trochanter of the femur and is a powerful flexor of the hip. Second is a muscle more posterior and lateral. This is the quadratus lumborum. These muscles are attached to the transverse processes of most lumbar vertebrae so will be posterior to the psoas major.
Our final group of muscle at this fourth lumbar vertebral level are the deep back muscles. Today, we'll be looking at four pairs of muscles working our way from medial to lateral as we identify them. These include the interspinales muscles, the multifidus muscles, the longissimus thoracis muscle, and the iliocostalis lumborum. We'll also take a look at the thoracolumbar fascia.
Most medially, closest to the spinous process of L4 is the interspinales. This muscle runs between the adjacent vertebral spines and can be seen slightly on either side of the spinous process – the spinous process of L4 in this cross-sectional image. The muscles are postural muscles.
The next muscle highlighted now is more lateral to the spinous processes. These muscles are the multifidus muscle. In the lumbar area, the multifidus muscles originate on the mammillary processes of the lumbar vertebrae and insert on the vertebral spines of the vertebrae situated three to five segments superiorly. Therefore, at any one level, they'll be several multifidus muscles traveling so that's why we see quite a thick portion of muscle highlighted in the cross-sectional image. Because of their oblique path, when they contract, they rotate the trunk.
Again, moving laterally, we'll come across the next muscle in this cross-sectional image. The longissimus muscle is one muscle split into three subdivisions. At the lumbar level – the subdivision we can see is the longissimus thoracis. This muscle is the longest of the back muscles. When these muscles on both sides of the spine work together, they extend the lumbar vertebrae, however, if only one side contracts, they'll laterally flex the vertebral column.
The final and most lateral of these deep back muscles is the iliocostalis lumborum. Similar to the longissimus, the iliocostalis is one group of muscles divided into three parts. At the lumbar level, the path that we can see is the iliocostalis lumborum. As it runs superiorly, it inserts into the angles of the ribs which is the most lateral attachment point of this group of muscles.
The last structure we'll identify in this section is the structure we can now see highlighted in green. This is the thoracolumbar fascia. Bounding the back muscles superficially and posteriorly, this fascia is the outermost layer of muscle or fibrous tissue in the lumbar region of the back.
Now that we've covered all of the muscles at the L4 level, we'll move on to the organs.
In this cross-sectional image, we'll be able to see the ascending colon, the descending colon, the ileum, and the ureters. The first organ we'll look at is the ascending colon. In the cross-sectional image, we can see that most of what is highlighted is actually empty space. This is the lumen of the ascending colon. In this image on the right of the screen, we can see part of the ascending colon highlighted in green. In order to see the ascending colon on this image, all that was done was an opening of the anterior abdominal wall. If we compare the two images to each other, we can see in each case the close proximity of the ascending colon to the anterior abdominal wall muscles.
Looking at the patient's left side in the cross-sectional image, we can now see the highlighted luminous descending colon. Again, it's closely related to the anterior abdominal wall, however, in this image, the greater omentum has been reflected and the small intestine has been removed. So what should be noticed and what can be confirmed by the cross-sectional image is that the descending colon is tucked a little bit further posteriorly compared to the ascending colon.
The part of the small intestine that is anterior and medial to the descending colon at the L4 level is the ileum which we can now see highlighted in green. As we know, the ileum is a long tube that twists and turns throughout the inferior abdomen so in the cross-sectional images, we see portions of the wall of the ileum as well as its lumen.
The structures that are now highlighted in green are small luminous structures. These are the ureters. The left ureter is the one we can see in the zoomed-in portion of the image. Note in both of these images the close proximity of the ureters to the psoas major muscles. They run inferiorly towards the bladder along the anterior surface of the psoas major.
Next stop are the nerves. There's only one nervous structure that we can identify in this cross-sectional image. It's the green highlighted structure located within the vertebral canal. Since we're at the level of L4, it's no longer the spinal cord as the spinal cord ends at L1 to L2 in adults. Instead, it is the cauda equina which consists of three or four lumbar spinal nerves, the sacral spinal nerves, and the coccygeal spinal nerves. These nerves are continuing down the vertebral canal inferior to the termination of the spinal cord to exit the vertebral canal at the appropriate level.
The final group of structures we'll identify are the arteries and veins seen at the L4 vertebral level. In this cross-sectional image, we'll be able to see the abdominal aorta, the inferior vena cava, the superior mesenteric artery, and the superior mesenteric vein.
This luminous structure right along the midline anterior to the vertebral body is the abdominal aorta. The fourth lumbar vertebral level is often where the abdominal aorta bifurcates into the right and left common iliac arteries. So depending on the exact location of the cut, you may see two arteries here in front of the vertebral body.
To the anatomical right of the abdominal aorta is the inferior vena cava. Although veins are luminous structures, they don't have the same elastic structure that arteries do and so often collapse in cadaveric images which is what we can see in this cross-sectional image.
Within the parts of the ileum that we can see at this level are both the superior mesenteric artery and the superior mesenteric vein. In these images, the artery is highlighted. The superior mesenteric artery supplies structures of the midgut so at this level will have already given off several branches and is therefore quite small in diameter at L4. Similarly, the superior mesenteric vein which we now see highlighted in green is connecting venous blood from the midgut. At the level of L4, only a few tributaries have contributed to the superior mesenteric vein so its diameter again will be quite small.
So when like someone needs to have a look at a CT scan at the L4 vertebral level? That's what we're going to be looking at next.
Colon cancer is the most common cancer of the gastrointestinal tract. A regularly used method to check for cancers is by performing a procedure called CT colonography. A CT scan is performed by the machine you can see in the image. In the scan, you would normally see a cross-section of the body at the level of the colon which, if you remember, are these parts just here – the ascending colon just here and the descending colon just here. If a polyp which can be a precursor to a cancer is viewed on the scan, it would pop as a shadow like this image here.
That brings us to the end of the tutorial. Before I let you go, let's have a quick review of what we looked at today. We began by looking at the musculature of the abdomen that we can see at the L4 level. This included muscles of the anterior abdominal wall – the external oblique muscles which are the most external muscles, the internal oblique muscles which lie just deep to the external obliques, the transversus abdominis which are deepest and have a transverse muscle fiber direction, and rectus abdominis which is enclosed by the rectus sheath. In the midline is the midline fusion of the aponeurosis of the external oblique, the internal oblique and the transversus abdominis muscles. This midline fusion is called the linea alba. And, finally, on either side of the rectus abdominis are the linea semilunaris which are again fusions of the aponeurosis of the muscles listed above.
We then looked at muscles of the posterior abdominal wall which are the psoas major – a powerful hip flexor – and quadratus lumborum which lies posterior and lateral to the psoas major. The final group of muscles we identified were the deep back muscles which include the interspinales running between the spinous processes of the vertebrae, the multifidus which travel more laterally and therefore rotate the trunk, the longissimus thoracis – extensors of the lumbar vertebrae – and the iliocostalis lumborum which runs most laterally inserting on the angle of the ribs.
Next, we identified the organs that are at the L4 level. These were the ascending colon located on the right side of the patient and the descending colon located on the left side of the patient. Then we saw portions of the wall and lumen of the twisted curvy ileum. And, finally, we looked at the ureters which carry urine from the kidneys to the urinary bladder.
Finally, we identified the visible neurovascular structures in cross-section at the L4 level. These structures were the cauda equina, the abdominal aorta, the inferior vena cava, the superior mesenteric artery, and the superior mesenteric vein.
That brings us to the end of our tutorial on the cross-section at the L4 vertebral level. I hope you enjoyed it and thanks for joining me.